1. Field of the Invention
This invention relates to a transmitting type liquid crystal displaying apparatus of a multi-system displaying television video of a plurality of systems that can differ in the number of horizontal scanning lines contained in the video.
2. Related Art Statement
A multi-system television receiver which can receive a plurality of television broadcasts which differ in the broadcasting system used is already known. Various standards are provided for the broadcasting system and are classified, for example, into a PAL system, SECAM system and NTSC system by the difference in the color broadcasting system. The PAL system and NTSC system differ from each other in the number of horizontal scanning lines. The number of horizontal scanning lines per field is 625 lines/2=312.5 lines in the PAL system and is 525 lines/2=262.5 lines in the NTSC system. For the multi-system television receiver receiving television signals of both PAL and NTSC systems, there is known a liquid crystal television receiver using, for example, a transmitting type liquid crystal displaying panel in the displaying part.
The liquid crystal displaying panel is formed of a pair of glass plates sealed with a liquid crystal between them pixel electrodes are formed like a matrix on one glass plate and common electrodes are formed on the other glass plate. Pixels are formed in the positions of the pixel electrodes. In an active matrix system, TFT's (Thin Film Transistors) are formed in the respective pixels, the gates of the respective TFT's are connected to respective gate lines wired in the horizontal direction of the panel, the drains are connected to respective data lines wired in the vertical direction of the panel and the sources are connected to the pixel electrodes. The TFT's are switched on by a gate signal (mentioned also as a Y driving signal hereinafter) at a high level (mentioned also as "H" hereinafter) fed to the respective gate lines and pass video signals fed through data lines to pixel electrodes so that a twisted nematic liquid crystal between the pixel electrodes and common electrodes may be driven, the light transmittivity may be controlled and videos may be displayed.
Now, as described above, the NTSC system and PAL system differ from each other in the number of horizontal scanning lines. However, in the case where both of these systems are made displayable, it is possible to use a liquid crystal displaying panel, which corresponds to the number of horizontal scanning lines, for example, of the PAL system. However, in case videos of the NTSC system are displayed in this liquid crystal displaying panel, a blank part in which no video is displayed will be produced in a part of the display. FIG. 6 is an explanatory view of the related art.
The number YP of pixels in the vertical direction of a liquid crystal displaying panel 71 is based on the number of scanning lines of the PAL system but on the other hand the number YN of pixels in the vertical direction required to display videos of the NTSC system is YN={YP--(312.5-262.5)}=YP--50 pixels. Also, if the number of pixels in the horizontal direction corresponding to the video of the PAL system is represented by XP, the number XN of pixels in the horizontal direction of the NTSC system will be XN=YN XP/YP. That is to say, if the video of the
system is to be displayed in the entire region of the liquid crystal displaying panel 71, the video of the NTSC system will be displayed in a region 72 and a vacant region (blank part) 73 will be produced and will occupy about 16% in the horizontal direction and about 50 lines in the vertical direction of the display.
When a video signal of the NTSC system is received, the respective pixels of this vacant region 73 will not be fed with the video signal, will be non-loaded and will be impressed with 0 potential or a direct current voltage (mentioned as a DC voltage hereinafter) on a predetermined level. For ease in visualizing the drawing, a predetermined black level will be applied to the respective pixels of the vacant region 73. However, if a DC voltage is applied to the liquid crystal for a long time, the liquid crystal molecules will deteriorate and will no longer have a rotary polarization corresponding to the impressed voltage. In case 0 potential is applied to the respective pixels of the vacant region 73, the deterioration of the liquid crystal molecules will be so much different between the vacant region 73 and the region 72 that the boundary between the regions 72 and 73 will be conspicuous.
For this reason, in a liquid crystal television receiver of a multi-system displaying videos of the PAL system and NTSC system, the number of pixels of the liquid crystal displaying panel has been set on the basis of the number of horizontal scanning lines of the NTSC system. In the video of the PAL system, the horizontal scanning lines are thinned at a rate of one out of 6 to 8 lines. That is to say, the horizontal scanning lines of the
system are thinned by about 50 lines so as to coincide with the number of horizontal scanning lines of the NTSC system so that the video of either of the NTSC system and PAL system may be displayed in the entire displaying region of the liquid crystal panel.
FIG. 7 is a block diagram showing a related art of a liquid crystal displaying apparatus 81 adopted in such a multi-liquid crystal television receiver.
The number of vertical direction pixels of a liquid crystal displaying panel 83 is set on the basis of the number of vertical effective scanning lines of the NTSC system and the number of horizontal direction pixels is limited by the capacity of a drive integrated circuit. Today, for a 4-inch size liquid crystal displaying panel, there are known displays having vertical 234.times.horizontal 479 pixels or vertical 220.times.horizontal 480 pixels. In the liquid crystal panel 83, gate lines (not illustrated) are wired in the horizontal direction, data lines (not illustrated) are wired in the vertical direction and pixels are formed at the intersections of these gate lines and data lines. The respective gate lines are serially fed with driving signals from Y drivers 87 and the data lines are fed with video signals from X drivers 88. The liquid crystal displaying panel 83, Y drivers 87 and X drivers 88 are controlled by a controlling circuit 82. By the way, the X drivers 88 and controlling circuit 82 are fed with a power source voltage through a terminal 86.
A compound synchronizing signal and NTSC/PAL switching signal are given to the controlling circuit 82 respectively through input terminals 84 and 85. The controlling circuit 82 makes a Y driving signal and X driving signal respectively on the basis of the compound synchronizing signal and NTSC/PAL switching signal and feeds them respectively to Y drivers 87 and X drivers 88. A common electrode driving signal generating circuit 89 is controlled by the controlling circuit 82 and generates a common voltage to be applied to the common electrodes of the liquid crystal panel 83 from the common electrode driving signal from a terminal 90. The controlling circuit 82 also makes an internal horizontal synchronizing signal and internal vertical synchronizing signal. On the basis of these internal horizontal and vertical synchronizing signals, by a polarity reversing circuit (not illustrated), the video signal is reversed, for example, at intervals of one horizontal period or vertical period and is input into an input terminal 93. A Y driver power source circuit 92 is fed with a Y driver power source voltage from a terminal 91 and generates a power source voltage to be fed to the Y drivers 87.
The video signal input through an input terminal 93 is fed to the X drivers 88 which sample the video signal from the input terminal 93 in a sampling period based on the controlling signal from the controlling circuit 82 and hold pixel data for one line of the liquid crystal displaying panel 83. That is to say, the NTSC system and PAL system are differ from each other in the sampling frequency. In the X driver 88, in case a video signal of either of the PAL system and NTSC system is input, the same number of pixel data will be sampled in one horizontal period. On the other hand, in the Y driver 87, on the basis of the Y driving signal from the controlling circuit 82, as synchronized with one horizontal period of the video signal, the gate lines of the liquid crystal displaying panel 83 will be sequentially switched on.
Now, a video signal of the NTSC system shall be displayed. On the basis of an NTSC/PAL switching signal from the terminal 85, the controlling circuit 82 sets the sampling frequency of the X driver 88 and controls the scanning of the Y driver 87 so that the Y driver 87 may shift the driving signal, for example, in one horizontal period and may switch on all the gate lines in one vertical scanning period. From the X driver 88, the pixel data of the respective lines are output to the data lines and, within one vertical scanning period, all the pixels of the liquid crystal displaying panel 83 are written in.
On the other hand, the video signal of the PAL system shall be displayed. The controlling circuit 82 varies the sampling frequency of the X driver 88 which holds the video data for the number of pixels of one line of the liquid crystal displaying panel 83 within one horizontal period. The Y driver 87 is controlled by the controlling circuit 82 and does not shift but switches off the driving signal only once within 6 to 8 horizontal periods and the video is not written in for only one horizontal period of every 6 to 8 horizontal periods. That is to say, in the PAL system, the horizontal scanning lines are thinned at a rate of one line out of 6 to 8 lines and the picture is compressed in the vertical direction. After all, about 50 lines are removed in one vertical scanning period and, the same as the video of the NTSC system, the video of the PAL system is also displayed in the same displaying region (entire region).
However, in this displaying system, as the horizontal scanning lines are thinned in the PAL system, the transmittivity of the PAL system is lower than of the NTSC system and the picture quality deteriorates in the PAL system reception.